You should be able to export it into something like Blender. Blender has a shelling operation. I think it's called solidify (under "object:mesh:faces:solidify"). On the right, there should be a slider/number input for how thick you wish the walls to be (make sure not to input backwards, else it will GROW out! ).

I can talk you through the full export/import and buttons in Blender etc if you wish to do it.